FIG. 1 shows a working machine of a hydraulic power shovel comprising a boom 1, an arm 2, a bucket 3, a boom cylinder 4, an arm cylinder 5, and a bucket cylinder 6. The boom 1, the arm 2, and the bucket 3 are moved by extending or contracting the cylinders 4, 5, or 6, respectively, causing a distal end of the bucket 3 to follow a predetermined path for excavation.
Conventionally, there are two modes of operation in automatic excavating path correction work by a hydraulic power shovel on a slope. In one mode (nose-fixed mode), as shown in FIG. 2A, the axes of the boom 1 and the arm 2 are activated in an interrelated manner to make the bucket nose excavate and finish a flat surface. In the other mode (fixed-angle to the ground mode), three axes, namely, the boom 1, the arm 2, and the bucket 3 are activated in an interrelated manner to perform excavation and surface finishing using a bottom surface of the bucket, as shown in FIG. 2B. Before beginning automatic operation, an operator must use a switch or the like to select between these two modes.
A prior art system for automatically selecting between the modes is disclosed in Japanese Patent Laid-Open No. 2-47432 publication, wherein a boom angle .theta..sub.1, an arm angle .theta..sub.2, a bucket angle .theta..sub.3, a body inclination .theta..sub.0, and a target excavating grade .theta., shown in FIG. 3 are inputted. An angle .beta. between the bottom surface of the bucket 3 and the flat surface, which is to be excavated, is determined at the beginning of the automatic operation from formula (1) below, and the computation result is compared with a predetermined value, thereby automatically determining the mode.
ti .beta.=3/2.pi.-(.theta..sub.0 +.theta..sub.1 +.theta..sub.2 +.theta..sub.3 +.theta.+.alpha.) (1)
(where e is the nose angle of the bucket).
Generally, in the case of the bucket used for the hydraulic power shovel, a standard tooth bucket shown in FIG. 4A needs to be replaced by various special buckets according to the particular work. On the other hand, however, a slope finishing bucket, shown in FIG. 4B, comes in an infinite number of shapes with more buckets being produced at general iron works, than genuine buckets produced by construction equipment manufacturers. Buckets produced by general iron works vary in dimensions from one bucket to another except for pin intervals of the buckets. In other words, the use of a method, wherein the mode is determined by determining the angle to the ground .beta. of the bucket bottom surface, poses a problem in that the nose angle .alpha. of the bucket must be corrected each time the bucket is changed except when a bucket having a predetermined shape is used.
Further, a problem arises when automatic operation is performed for purposes other than excavation. For example, if the position of a hook is linearly moved in suspension work, as shown in FIG. 5, the automatic determination by the described mode determination method erroneously selects the nose-fixed mode because of a significant difference between the target direction of movement and the orientation of the angle .beta. between the bottom surface of the bucket and the ground. This presents a problem in that the nose point moves as indicated by a solid line rather than moving along the path intended by the operator and indicated by a broken line.
Hence, in order to maintain the current angle to the ground .beta. when the direction of the movement of the working machine is given, the bucket 3 must be turned either to a dump truck side or an excavating side. For instance, as shown in FIG. 6A, if the angle of movement of the bucket 3 on the excavating side is small, then it soon becomes impossible to maintain the angle to the ground .beta. in the fixed-angle to the ground mode. Therefore, it is very likely that the operator's intention is the nose-fixed mode. On the other hand, if a bucket attitude angle .gamma., which is the attitude of the bucket 3 with respect to the arm 2, is large as shown in FIG. 6B, then the resulting path partially extends beyond (as shown by a hatched area) an arc drawn by the bucket nose point in the nose-fixed mode wherein the arm 2 is turned without moving the bucket 3 with respect to the arm 2. Therefore, a target excavating surface is ruined in the hatched area during automatic operation. Hence, it is very likely that the operator's intention in this case is the fixed-angle to the ground mode. Therefore, it is necessary to calculate these two possibilities and determine the automatic operation mode according to the magnitude of the calculated values.
Furthermore, an example of a prior art automatic operation in a power shovel is disclosed in Japanese Patent Laid-Open No. 2-221527 publication. It comprises an actuator controlling means, which controls actuators for an excavator; a working machine attitude detecting means, which detects the attitude angles of the boom, arm, and tip working machine of an excavating machine; a grade input means, which gives a target excavating grade for a surface to be excavated by the tip working machine; a distal end inclination input means, which gives a target inclination of the tip working machine with respect to a reference plane; and an actuator operating amount computing means, which computes an operating amount for moving the tip working machine at a determined specific speed with the given inclination and the given excavating grade in response to a detected value received from the working machine attitude detecting means and command values received from the grade input means and the tip inclination input means, and supplies the computed value to the actuator controlling means.
However, in such a control apparatus, it is necessary to specify input signals including a grade input, a control inclination input, and an excavating direction input for the excavating conditions of a slope surface at the time the automatic operation is begun. Furthermore, there is a problem in that the need for entering the input signals is easily forgotten, and all inputs must be checked for correctness each time before the automatic operation is started.